1. Field of the Invention
The present invention pertains to a linear actuator and, more particularly, a differential linear actuator having first and second components which are adapted to linearly shift in opposite directions in response to rotation of an input member in order to reduce the torque that must be counteracted by the input member.
2. Discussion of the Prior Art
Linear actuators are widely known in the art. A basic type of known linear actuator includes a fixed actuator housing within which is rotatably mounted a drive sleeve. An output member of the linear actuator is fixedly secured at one end to an element adapted to be shifted by the linear actuator and is also connected to the drive sleeve through a rotary to linear conversion connector such that rotation of the drive sleeve causes axial shifting of the output member. One type of known arrangement for causing this rotary to linear movement is a recirculating ball-type screw assembly. In such known linear actuator arrangements, the length at which the actuator can linearly shift the adjustable element is generally determined by the length of the output member. In addition, all of the torque exerted on the output member must be counteracted by the drive sleeve.
In another known type of linear actuator arrangement, the drive sleeve is connected through a plurality of splines to an intermediate, axially shiftable actuator member such that rotation of the drive sleeve causes rotation of the intermediate actuator member. The intermediate actuator member is also connected through a rotary to linear connection assembly to a fixed portion of the linear actuator such that rotation of the intermediate actuator member causes the member to axially shift. In addition, an output member of the linear actuator is further connected to the intermediate actuator member through a second rotary to linear conversion connection such that rotation of the intermediate actuator member also causes the output member to shift in the same direction as the intermediate actuator member. By this arrangement, since both the intermediate actuator member and the output member shift axially in the same direction, the linear actuator can produce a longer output stroke. In this prior art arrangement, the torque transmitted to the drive sleeve equals the combined torque exerted on the intermediate actuator member through the output member and the fixed portion of the linear actuator. In other words, since both the intermediate actuator member and the output member extend in the same direction, the torque reacted by the intermediate actuator member at both of the rotary to linear conversion connections combine to a high resultant torque which is transmitted to the input drive sleeve through the spline connection.
In some environments, such as landing flaps and other actuation systems used in military and commercial aircraft, the torque levels developed during use of such linear actuator arrangements are rather high and since the drive sleeve must counteract these torque values, the drive sleeves have to be made from strong, rather heavy materials in order to prevent system failure. In addition, there are various environments, such as in landing flap controls mentioned above, wherein only a relatively small amount of axial shifting is required, at high torque values, to move the control component throughout its entire operating range. Therefore, there is a need in some environments for a linear actuator which can shift a control component within a rather small operating range but which does not require the input drive sleeve to counteract a high torque load.